The present invention relates to an improved means for aligning a closed loop interferometric fiber optic gyroscope. Fiber optic gyroscopes are used to sense rotation. Closed loop interferometric fiber optic gyroscopes (IFOGs) sense angular rate by propagating light in counter-rotating directions around a fiber optic spool as shown in FIG. 1. The fiber optic spool is wound around the axis in which rotation is to be sensed. A broadband light source (1) injects light through a coupler (2), into an integrated optics chip (IOC) (3). In the IOC, the light is split into two paths and injected in counter-rotating directions through the fiber spool (5). The optical fiber has a typical length of 100 to 2,000 meters. Rotation of the fiber optic spool around the axis in which rotation is to be sensed causes an effective optical path length increase in one direction with a corresponding path length decrease in the other direction. Consequently, the light propagating through the fiber spool is shifted in phase in accordance with a phenomena known as the Sagnac Effect. The light wave traveling in the rotation direction acquires a phase lead, while the opposite traveling wave acquires a phase lag. The composite phase shift is proportional to the rotation rate that is desired to be measured. Small rotation rates produce small phase shifts, while large rotation rates produce large phase shifts. The light wave is then routed back through the IOC (3) and the coupler (2) to an interferometric phase detector. The counter-rotating light beams produce an optical interference pattern at the phase detector (6), with phase proportional to the rotation rate of the fiber spool (5).
As shown in FIG. 3(b), the interference pattern at the phase detector follows a raised-cosine pattern. This interference pattern is caused by the difference in phase of the two light waves. The difference in phase is proportional to the rotation rate. That is, the output of the phase detector is proportional to the cosine of the phase difference between the two light waves. Consequently, the phase detector output has a small slope at small phase differences, small phase differences correspond to small rates of rotation. This means that the rate of change of the cosine function near zero will be very small. As a result, the output function provides very low sensitivity for low rotation rates.
Therefore, it is desirable to move the operating point of the phase detector from 0.degree. to .+-.90.degree. (.+-..PI./2 Rad), where the detector has high sensitivity. This is accomplished by adding a phase shift to one of the light waves. That is the function of the phase modulator (4). The phase modulator (4) (see, e.g., FIG. 2) is a serrodyne device that generates a phase shift proportional to the change in applied voltage at its input. The applied voltage is the sum of the voltage produced by the bias modulator and the serrodyne ramp generator.
The purpose of the biasing modulator is shown in FIG. 3(b). It is used to shift the voltage sampling point out to approximately .+-.90.degree. (.+-..PI./2 Rad) where the voltage slope is maximized and, consequently, the sensitivity of the modulator is maximized. This allows the phase of the interference pattern to be accurately computed by differencing the voltage measured at the positive and negative bias points (which equals the slope of the raised-cosine curve at .PI./2 Rad). Therefore, small variations in rotation rate can be accurately detected and will vary linearly since the slope of the curve around .+-..PI./2 Rad is linear.
The biasing modulator delays one of the waves by .PI./2 Rad with respect to the other. It does this by adding or subtracting a voltage corresponding to .PI./2 to the input of the phase modulator (4). As a result, the detector has its maximum sensitivity for low rotation rates. In addition, this sensitivity is close to being linear.
However, as the rotation rate increases, the response of the detector moves away from .PI./2, where the curve becomes less and less linear. In addition, the curve's slope flattens, thereby reducing the detector's sensitivity.
Therefore, a second modulation component is added. The other modulation component is the serrodyne ramp modulator, which applies the phase shift necessary to hold the gyro in a rebalanced condition. It provides sufficient phase change so as to cancel the phase shift difference generated by the opposite traveling light waves due to rotation.
As shown in FIG. 3(c), in order to apply a constant phase offset, the control voltage input to the phase modulator (4) must be applied as a stepped ramp which changes, each Eigen period, by an amount proportional to the desired phase shift. Since the required re-balance phase shift can be large or small, positive or negative, depending on the gyroscope's motion, the stepped ramp is correspondingly fast or slow, increasing or decreasing, as required to produce the phase shift needed to rebalance the gyroscope.
The ramp can not increase indefinitely and, at some point, must fly-back to zero. The flyback is made transparent to the gyroscope's phase modulator (4) by scaling the magnitude of the flyback voltage to correspond to the desired step size plus or minus 360.degree. (step size .+-.2.PI. Rad). This produces the same phase shift during the fly-back as is produced by the other voltage steps in the ramp, effectively making a continuous phase shift.
The composite phase modulation signal is illustrated in FIG. 3(d). This signal is the sum of the biasing modulation (a) and the serrodyne ramp modulation (c).
Therefore, closed loop operation is obtained by measuring the detector phase (6) and driving a matching phase shift into an electro-optic phase modulator (4), thus re-balancing (i.e., zeroing) the detected phase shift.
The phase shift necessary to hold the gyroscope in a rebalanced condition is directly proportional to angular velocity of the fiber spool, and the processing electronics (7) derive the rate output (19) of the gyroscope from the commanded phase shift. FIG. 2 illustrates the best prior art for the closed loop processing electronics (7). The phase detector (6) output (which in a preferred embodiment is an analog square wave) is first amplified (9) and then digitized with an A/D converter (10). The digitized output of the A/D converter is a modulated signal, which is de-modulated (11) to recover the detector phase error. (In a preferred embodiment the demodulator (11) is a square wave detector). The phase error is then integrated in a digital accumulator (12) or integrator to compute the present gyro rate. The output from the accumulator or integrator (12) corresponds to the rotational rate of the gyroscope and drives both a serrodyne modulated ramp generator (13), and the gyroscope rate output (19).
The serrodyne modulation ramp generator (13) produces a stepped digital waveform in which the step size corresponds to the amount of phase shift required to hold the phase detector (6) output at null (gyroscope phase held in a re-balanced condition). The digital output of the ramp generator (13) corresponds to the rotational rate of the gyro and is converted to a stepped analog voltage ramp by a D/A converter (14). The voltage ramp is summed (18) with a biasing modulation voltage to form a composite phase modulation signal, which is amplified (15) and used to drive the phase modulator (4). The phase modulator adds a phase shift to one of the light waves to re-balance the gyro for closed loop operation.
In a preferred embodiment, the biasing modulation voltage is generated with a square wave generator (17), which is slaved to the same bias modulation timing (16) that provides a reference to the demodulator (11).
The components of the composite phase modulation signal are illustrated in FIG. 3. As shown in FIG. 3(a), in a preferred embodiment, the biasing modulation is a square wave, switched at the gyro Eigen period. The Eigen period is simply the time required for light to propagate around the fiber spool.
Since the commanded re-balance phase shift is the gyroscope output, the linearity and stability of the transfer function between the digital phase command at the ramp generator (13) and the actual phase shift produced by the modulator (4), directly impacts gyro performance. In order to produce a high performance IFO gyroscope, this phase command transfer function must be held stable despite changes in environmental factors such as temperature, humidity, power supply voltage, etc.
The present state-of-the-art for stabilizing the phase command transfer function is embodied in a technique that utilizes a gain scaling control loop to regulate the transfer function and hold it constant despite environmental disturbances. This technique consists of regulating the gain of the serrodyne ramp modulation so as to hold the fly-back transition equal to exactly .+-.360.degree. (plus the appropriate phase step for re-balance).
As shown in FIG. 2, the D/A converter (14) for the serrodyne ramp modulator has its gain controlled by a separate modulation gain-scaling D/A converter (21). This D/A (21) scales the magnitude of the serrodyne ramp and can scale the ramp magnitude such that the ramp Fly-back voltage produces either greater or lesser than 360.degree. of phase shift at the phase modulator (4).
The gain-scaling D/A (21) is controlled by a modulation gain accumulator (20) which accumulates the phase error (6,9,10,11) produced when ramp flyback occurs. The modulation gain accumulator (20) is normally held at its accumulated value, and is enabled to run only when the ramp generator (13) signals that a ramp flyback is about to occur. Once enabled, it samples phase error (output of demodulator (11)) for two Eigen periods, so that both phases of biasing modulation are accumulated.
The gain scaling control loop formed by the modulation gain accumulator (20) and the modulation gain-scaling D/A (21) operates to stabilize the phase command transfer function by adjusting the gain of the serrodyne ramp modulation D/A (14) until the phase error measured during a fly-back event is zero, which corresponds to a phase shift of .+-.360.degree.. If the gain (20,21) is slightly in error, then the fly-back phase shift will not be precisely .+-.360.degree., and a non-zero phase error will be accumulated (20). This corrects the ramp magnitude (21,14), driving it to a balanced condition in which the ramp flyback is forced to a precise .+-.360.degree..